The Night Sky in Transition: From Summer to Fall

This sky map shows the location of the bright stars Vega, Altair and Deneb in the eastern night sky in summer 2012. The stars form the Summer Triangle visible in North Hemisphere night skies. Credit: Starry Night Software

The night sky's autumn constellations follow the summer stars so closely they can be seen well before the air gets chilly. As we move into the final days before the official arrival of autumn here in the Northern Hemisphere on Sept. 22, skywatchers can now see a sky in transition.

Many of the striking star groups and rich Milky Way fields of a summer evening still remain with us in the western sky, while the brilliant star Capella ascending above the northeast horizon is a promise of more luminaries to come. Indeed, in another six to eight weeks, Orion and his neighbors will be dominating the evening skies, reminding us of the approaching winter season.

Still very well placed is the "Summer Triangle," a roughly isosceles figure composed of three first-magnitude stars, Vega, Altair and Deneb.

Some may wonder why we call it the Summer Triangle since it’s nearly overhead during the late evening hours as summer transitions into fall. But during the summer months, the Triangle is visible from dusk to dawn, whereas during autumn, this configuration sinks lower and lower in the west. [Amazing Night Sky Views of Sept. 2012]

During the mid- and late-evening hours, the stars of the autumn season cover much of the eastern and southern sky. This whole area has been called the "Celestial Sea," because many of the constellations have an association with water.

For example, looking toward the south-southeast reveals one member of this watery fraternity, the only one whose name is attached to a popular song: "Aquarius." And yet how many who have heard the song know what Aquarius really is?

This zodiacal star pattern traditionally represents a man holding a water jar (marked by an inverted Y-shaped group of four stars), which is spilling a vaguely marked stream of water southward into the mouth of Piscis Austrinus, the Southern Fish.

Across southern Canada, the northern United States and much of Europe, the stars that make up this constellation usually cannot be seen. Although above the horizon, they are too low to penetrate the horizon haze. At more southerly latitudes, these stars are much higher up in the sky, though still quite dim.

The constellation’s main star is much more conspicuous: silvery-white Fomalhaut, the 18th-brightest star in the sky and the only first-magnitude star in the whole collection of watery constellations. Indeed, Fomalhaut is the only true first-magnitude star of autumn. (Magnitude is a measure of brightness, with lower magnitudues denoting objects that appear more luminous in the sky. For comparison, the full moon has a magnitude of about minus 13.)

Fomalhaut lies in an empty region of the autumn skies, and it's sometimes referred to as "The Solitary One." It can be identified by extending a line along the western (right) side of the Great Square of Pegasus about three times its own length.

Fomalhaut is often described in observing books as "reddish," though it is probable that the effects of our atmosphere are responsible for this impression, as this star is always seen at a low altitude for northern observers.

Fomalhaut is Arabic for "mouth of the fish." This star, which lies about 25 light-years away, is about twice as wide as our sun and 19 times more luminous.

In her book “The Friendly Stars," Martha Evans Martin wrote: "The loneliness of this star, added to the somber signs of approaching autumn, sometimes gives one a touch of melancholy. In November and December, when the winter stillness has fallen upon us, a glance toward the southwest will discover Fomalhaut, still placid and alone."

And echoing what Martin wrote, I can only add that this is the time of the year when the days get shorter and the faces get longer.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.